Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

An object is to provide a light-emitting element capable of emitting light with a high luminance even at a low voltage, and having a long lifetime. The light-emitting element includes n EL layers between an anode and a cathode (n is a natural number of two or more), and also includes, between m-th EL layer from the anode and (m+1)-th EL layer (m is a natural number, 1≤m≤n−1), a first layer including a first donor material in contact with the m-th EL layer, a second layer including an electron-transport material and a second donor material in contact with the first layer, and a third layer including a hole-transport material and an acceptor material in contact with the second layer and the (m+1)-th EL layer.

TECHNICAL FIELD

The present invention relates to a light-emitting element havinglight-emitting layers between a pair of electrodes. In addition, thepresent invention relates to a light-emitting device in which thelight-emitting element is used and an electronic device and a lightingdevice in each of which the light-emitting device is used.

BACKGROUND ART

In recent years, a light-emitting element in which a light-emittingorganic compound or a light-emitting inorganic compound is used as alight-emitting material has been actively developed. In particular, alight-emitting element called an electroluminescence (hereinafterreferred to as EL) element, which has a structure in which alight-emitting layer containing a light-emitting material is providedbetween a pair of electrodes, has attracted attention as anext-generation flat panel display element because of itscharacteristics such as thinness, lightweight, high response time, anddirect-current low-voltage driving. In addition, a display in which anEL element is used has a feature that it is excellent in contrast andimage quality and has a wide angle of view. Moreover, because the ELelement is a plane light source, application to a light source such as abacklight of a liquid crystal display and lighting is considered.

An EL element includes a pair of electrodes and a light-emitting layerwhich contains a light-emitting material and is provided between thepair of electrodes. When current flows in the light-emitting layer, thelight-emitting material is excited, and then the EL element can emitlight of a predetermined color. To make a large amount of current flowin the light-emitting layer is effective in increasing the luminance ofthe EL element. However, application of a large amount of current intothe EL element increases power consumption. In addition, the applicationof a large amount of current in the light-emitting layer alsoaccelerates deterioration of the EL element.

In view of the above, a light-emitting element in which a plurality oflight-emitting layers are stacked has been proposed (e.g., PatentDocument 1). Patent Document 1 discloses a light-emitting element inwhich a plurality of light-emitting units (hereinafter in thisspecification, the light-emitting unit is also referred to as an ELlayer) are provided and the light-emitting units are separated by acharge-generation layer. More specifically, it discloses alight-emitting element in which a charge-generation layer formed ofvanadium pentoxide is provided on a metal-doped layer functioning as anelectron-injection layer of a first light-emitting unit, and further asecond light-emitting unit is provided over the charge-generation layer.The light-emitting element disclosed in Patent Document 1 can emit lightwith a higher luminance than a light-emitting element having onelight-emitting layer, when current having the same current density isapplied to the elements.

REFERENCE Patent Document

[Patent Document 1] Japanese Patent Laid-Open No. 2003-272860

DISCLOSURE OF INVENTION

It is an object of one embodiment of the present invention to provide alight-emitting element capable of emitting light with a high luminance.

It is another object of one embodiment of the present invention toprovide a light-emitting element with a long lifetime.

It is another object of one embodiment of the present invention toprovide a light-emitting element capable of low-voltage driving.

It is another object of one embodiment of the present invention toprovide a light-emitting device with low power consumption.

It is another object of one embodiment of the present invention toprovide an electronic device or a lighting device with low powerconsumption.

One embodiment of the present invention is a light-emitting elementincluding n EL layers between an anode and a cathode (n is a naturalnumber of two or more) and also includes, between m-th EL layer from theanode and (m+1)-th EL layer (m is a natural number, 1≤m≤n−1), a firstlayer including a first donor material and being in contact with them-th EL layer, a second layer including an electron-transport materialand a second donor material and being in contact with the first layer,and a third layer including a hole-transport material and an acceptormaterial and being in contact with the second layer and with the(m+1)-th EL layer.

Another embodiment of the present invention is a light-emitting elementincluding n EL layers between an anode and a cathode (n is a naturalnumber of two or more) and also includes, between m-th EL layer from theanode and (m+1)-th EL layer (m is a natural number, 1≤m≤n−1), a firstlayer including a first electron-transport material and a first donormaterial and being in contact with the m-th EL layer, a second layerincluding a second electron-transport material having a LUMO level whichis lower than a LUMO level of the first electron-transport material anda second donor material and being in contact with the first layer, and athird layer including a hole-transport material and an acceptor materialand being in contact with the second layer and with the (m+1)-th ELlayer.

Another embodiment of the present invention is a light-emitting devicemanufactured using the above-described light-emitting element.

In addition, another embodiment of the present invention is anelectronic device including the above-mentioned light-emitting device.

Furthermore, another embodiment of the present invention is a lightingdevice including the above-mentioned light-emitting device. Note thatthe term “lighting device” in this specification means a light sourcewhich can control lighting and non-lighting, and the purpose of which isto enable people to have better lives with light. For example, with theuse of light, a scene, a visual object, and the periphery thereof arelit up to be more recognizable, or information is transmitted with avisual signal.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify theinvention.

The light-emitting element of one embodiment of the present inventionhas a plurality of EL layers, and accordingly, light emission with ahigh luminance is possible.

In addition, since the light-emitting element of one embodiment of thepresent invention has a plurality of EL layers, the lifetime in the casein which the light-emitting element emits light with a high luminancecan be improved.

Furthermore, the light-emitting element of one embodiment of the presentinvention has a structure capable of transporting carries between theplurality of EL layers favorably. Therefore, the driving voltage of thelight-emitting element can be reduced.

The light-emitting device of one embodiment of the present inventionincludes the light-emitting element with a reduced driving voltage,which results in a reduction in the power consumption of thelight-emitting device.

The electronic device or lighting device which is one embodiment of thepresent invention includes the light-emitting device with reduced powerconsumption, which can result in a reduction in the power consumption ofthe electronic device or the lighting device.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B respectively illustrate an example of an elementstructure of a light-emitting element and an example of a band diagramof the light-emitting element described in Embodiment 1;

FIGS. 2A and 2B respectively illustrate an example of an elementstructure of a light-emitting element and an example of a band diagramof the light-emitting element described in Embodiment 2;

FIGS. 3A and 3B respectively illustrate an example of an elementstructure of a light-emitting element and an example of a band diagramof the light-emitting element described in Embodiment 3;

FIGS. 4A and 4B illustrate examples of an element structure of alight-emitting element described in Embodiment 4;

FIGS. 5A and 5B respectively illustrate an example of an elementstructure of a light-emitting element and emission spectra thereofdescribed in Embodiment 5;

FIGS. 6A to 6C illustrate active-matrix light-emitting devices describedin Embodiment 6;

FIGS. 7A and 7B illustrate a passive-matrix light-emitting devicedescribed in Embodiment 6;

FIGS. 8A to 8E illustrate electronic devices described in Embodiment 7;

FIG. 9 illustrates lighting devices described in Embodiment 8;

FIGS. 10A to 10E illustrate lighting devices described in Embodiment 8;

FIGS. 11A and 11B illustrate element structures of light-emittingelements described in Examples 1 and 2;

FIG. 12 shows characteristics of the light-emitting elements describedin Example 1;

FIG. 13 shows characteristics of the light-emitting elements describedin Example 1;

FIG. 14 shows characteristics of the light-emitting elements describedin Example 2;

FIG. 15 shows characteristics of the light-emitting elements describedin Example 2;

FIG. 16 shows characteristics of light-emitting elements described inExample 3; and

FIG. 17 shows characteristics of the light-emitting elements describedin Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and examples of the present invention are detailed belowwith reference to the accompanying drawings. Note that it is easilyunderstood by those skilled in the art that the present invention is notlimited to the description below and that the modes and details can bemodified in various ways without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beconstrued as being limited to the description of embodiments andexamples below.

Embodiment 1

In this embodiment, one embodiment of a light-emitting element isdescribed with reference to FIGS. 1A and 1B.

A light-emitting element illustrated in FIG. 1A includes a first EL anda second EL layer 107 each including a light-emitting region between apair of electrodes (an anode 101 and a cathode 102). In addition, thelight-emitting element also includes, from the anode 101 side, anelectron-injection buffer layer 104 in contact with the first EL layer103, an electron-relay layer 105 in contact with the electron-injectionbuffer layer 104, and a charge-generation layer 106 in contact with theelectron-relay layer 105 and the second EL layer 107.

Purposes of the electron-injection buffer layer 104 are to reduce aninjection barrier in injection of electrons into the first EL layer 103,and to inject electrons more efficiently into the first EL layer 103. Inthis embodiment, the electron-injection buffer layer 104 is formedincluding a donor material.

The purpose of the electron-relay layer 105 is to immediately transferelectrons to the electron-injection buffer layer 104. In thisembodiment, the electron-relay layer 105 is formed including anelectron-transport material and a donor material. Note that theelectron-transport material used for the electron-relay layer 105 issuch a material that has a high electron-transport property and that itsLUMO (lowest unoccupied molecular orbital) level is between the LUMOlevel of the first EL layer 103 which is described in this embodimentand an acceptor level of an acceptor material in the charge-generationlayer 106. Specifically, a material having a LUMO level greater than orequal to −5.0 eV is preferably used as the electron-transport materialused for the electron-relay layer 105. Furthermore, a material having aLUMO level greater than or equal to −5.0 eV and lower than or equal to−3.0 eV is more preferably used as the electron-transport material usedfor the electron-relay layer 105.

The purpose of the charge-generation layer 106 is to generate holes andelectrons, which are carriers of the light-emitting element. In thisembodiment, the charge-generation layer 106 is formed including ahole-transport material and an acceptor material.

FIG. 1B illustrates a band diagram of the element structure illustratedin FIG. 1A. In FIG. 1B, reference numerals 111, 112, 113, 114, 115, 116,and 117 respectively denote a Fermi level of the anode 101, a Fermilevel of the cathode 102, the LUMO level of the first EL layer 103, adonor level of the donor material in the electron-relay layer 105, theLUMO level of the electron-transport material in the electron-relaylayer 105, the acceptor level of the acceptor material in thecharge-generation layer 106, and the LUMO level of the second EL layer107.

In FIG. 1B, electrons generated in the charge-generation layer 106transfer into the LUMO level of the electron-transport material in theelectron-relay layer 105. Further, they transfer through theelectron-injection buffer layer 104 to the LUMO level of the first ELlayer 103. After that, in the first EL layer 103, holes injected fromthe anode 101 and the electrons injected from the charge-generationlayer 106 recombine. As a result, the first EL layer 103 emits light. Asin the first EL layer 103, in the second EL layer 107, holes injectedfrom the charge-generation layer 106 and electrons injected from thecathode 102 recombine. As a result, the second EL layer 107 emits light.

The light-emitting element described in this embodiment includes theelectron-relay layer 105 containing an electron-transport material and adonor material. The donor material moves the LUMO level of theelectron-transport material to a lower energy level. The LUMO level ofthe electron-transport material in the electron-relay layer 105, whichis initially relatively low because it is lower than the LUMO level ofthe first EL layer 103, is further decreased by the donor material. As aresult, the barrier in acceptance of electrons by the electron-relaylayer 105 from the charge-generation layer 106 is reduced. In addition,electrons that the electron-relay layer 105 accepts are immediatelyinjected into the first EL layer 103 by the electron-injection bufferlayer 104 without generation of a large injection barrier. As a result,low-voltage driving of the light-emitting element is possible.

Next, specific examples of each material described above are given.

The donor material contained in the electron-injection buffer layer 104and the electron-relay layer 105 can be an alkali metal, an alkalineearth metal, a rare earth metal, or compounds of an alkali metal, analkaline earth metal, or a rare earth metal (including an oxide, ahalide, and carbonate). Specific examples include metals such as lithium(Li), cesium (Cs), magnesium (Mg), calcium (Ca), strontium (Sr),europium (Eu), and ytterbium (Yb) and compounds thereof. These metals ormetal compounds are preferable because their electron-injection propertyis high.

The electron-transport material contained in the electron-relay layer105 can be a perylene derivative, a nitrogen-containing condensedaromatic compound, or the like.

Specific examples of the perylene derivative include3,4,9,10-perylenetetracarboxylicdianhydride (abbreviation: PTCDA),3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (abbreviation:PTCBI), N,N-dioctyl-3,4,9,10-perylenetetracarboxylic diimide(abbreviation: PTCDI-C8H),N,N-dihexyl-3,4,9,10-perylenetetracarboxylicdiimide (abbreviation:HexPTC), and the like.

Specific examples of the nitrogen-containing condensed aromatic compoundinclude pirazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile(abbreviation: PPDN),2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT(CN)₆), 2,3-diphenylpyrido[2,3-b]pyrazine (abbreviation: 2PYPR),2,3-bis(4-fluorophenyl)pyrido[2,3-b]pyrazine (abbreviation: F2PYPR), andthe like. Note that a nitrogen-containing condensed aromatic compound ispreferably used as the electron-transport material contained in theelectron-relay layer 105 because of its stability. Furthermore, ofnitrogen-containing condensed aromatic compounds, a compound having anelectron-withdrawing group such as a cyano group or a fluoro group ispreferably used, in which case electrons are easily accepted in theelectron-relay layer 105.

Alternatively, it is also possible to use the following as theelectron-transport material contained in the electron-relay layer 105:perfluoropentacene, 7,7,8,8-tetracyanoquinodimethane (abbreviation:TCNQ), 1,4,5,8-naphthalenetetracarboxylicdianhydride (abbreviation:NTCDA), copper hexadecafluorophthalocyanine (abbreviation: F₁₆CuPc),N,N′-bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl-1,4,5,8-naphthalenetetracarboxylicdiimide (abbreviation: NTCDI-C8F),3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophen(abbreviation: DCMT), a methanofullerene (for example, [6, 6]-phenyl C₆₁butyric acid methyl ester), and the like.

The hole-transport material contained in the charge-generation layer 106can be any of a variety of organic compounds such as an aromatic aminecompound, a carbazole derivative, an aromatic hydrocarbon, and a highmolecular compound (such as an oligomer, a dendrimer, or a polymer).Most of the materials described here have a hole mobility of greaterthan or equal to 1×10⁻⁶ cm²/Vs.

Specific examples of the aromatic amine compound include4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),N,N-bis(4-methylphenyl)-N,N-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N′-diphenyl-[1,1′-Biphenyl]-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivative include3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like. In addition, the following can begiven: 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Specific examples of the aromatic hydrocarbon include2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene;tetracene; rubrene, perylene; 2,5,8,11-tetra(tert-butyl)perylene; andthe like. Further, the aromatic hydrocarbon may have a vinyl skeleton.As the aromatic hydrocarbon having a vinyl group, for example,4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi),9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA),and the like can be given.

Further, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA)can also be used as the hole-transport material.

The hole-transport material described above preferably has a holemobility of greater than or equal to 1×10⁶ cm²/Vs. Note that any othersubstance that has a higher hole-transport property than anelectron-transport property can be used.

In the case of employing an evaporation method for formation of theabove aromatic hydrocarbon, it is preferable that the number of carbonatoms that forms a condensed ring be 14 to 42 in terms of evaporativityat the time of evaporation or film quality after film formation.

As the acceptor material contained in the charge-generation layer 106, atransition metal oxide and an oxide of a metal belonging to Groups 4 to8 of the periodic table can be used. Specifically, metal oxides such asvanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide canbe given. These metal oxides are preferable because they have highelectron-accepting properties. In particular, molybdenum oxide ispreferably used as the acceptor material. Note that molybdenum oxide hasa feature of a low hygroscopic property.

Further, electron-injection buffer layer 104, the electron-relay layer105, and the charge-generation layer 106 can be formed by any of avariety of methods regardless of a dry process (e.g., a vacuumevaporation method or a sputtering method) or a wet process (e.g., anink-jet method, a spin coating method, or an application method).

Next, specific examples of the anode 101 and the cathode 102 which aredescribed above are given.

The anode 101 can be formed using a metal, an alloy, anelectrically-conductive compound, a mixture of these materials, or thelike, having a high work function (specifically, a work function ofgreater than or equal to 4.0 eV is preferable). Specifically, conductivemetal oxides such as indium oxide-tin oxide (ITO: indium tin oxide),indium oxide-tin oxide containing silicon or silicon oxide, indiumoxide-zinc oxide (IZO: indium zinc oxide), and indium oxide containingtungsten oxide and zinc oxide can be given.

Thin films of these conductive metal oxides can be formed by asputtering method. Alternatively, the films can be formed by a sol-gelmethod or the like. For example, a film of indium oxide-zinc oxide (IZO)can be formed by a sputtering method with use of a target in which zincoxide is added to indium oxide at 1 wt % to 20 wt %. Indium oxidecontaining tungsten oxide and zinc oxide can be formed by a sputteringmethod using a target in which tungsten oxide and zinc oxide are addedto indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %,respectively.

In addition, it is possible to use the following for the anode 101: gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd),titanium (Ti), a nitride thereof (e.g., titanium nitride), and an oxidesuch as molybdenum oxide, vanadium oxide, ruthenium oxide, tungstenoxide, manganese oxide, or titanium oxide. Alternatively, a conductivepolymer such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonicacid) (abbreviation: PEDOT/PSS) or polyaniline/poly(styrenesulfonicacid) (abbreviation: PAni/PSS) may be used. Note that, in the case inwhich a charge-generation layer is provided in contact with the anode101 as a part of the first EL layer 103, a variety of conductivematerials can be used for the anode 101 regardless of the magnitude oftheir work functions. Note that the charge-generation layer can have thesame structure as that of the charge-generation layer 106 providedbetween the first EL layer 103 and the second EL layer 107 describedabove.

The cathode 102 can be formed using a metal, an alloy, anelectrically-conductive compound, a mixture of these materials, or thelike, having a low work function (specifically, a work function of lessthan or equal to 3.8 eV is preferable). Specifically, the following canbe given: an element that belongs to Group 1 or 2 of the periodic table,that is, an alkali metal such as lithium (Li) or cesium (Cs), analkaline earth metal such as magnesium (Mg), calcium (Ca), or strontium(Sr), an alloy containing these elements (e.g., MgAg or AlLi), arare-earth metal such as europium (Eu) or ytterbium (Yb), an alloycontaining these elements, and the like. Note that a film of an alkalimetal, an alkaline earth metal, or an alloy thereof can be formed by avacuum evaporation method. Alternatively, an alloy containing an alkalimetal or an alkaline earth metal can be formed by a sputtering method.

Alternatively, the cathode 102 can be formed using a stack of a thinfilm of an alkali metal compound, an alkaline earth metal compound, or arare earth metal compound (e.g., lithium fluoride (LiF), lithium oxide(LiOx), cesium fluoride (CsF), calcium fluoride (CaF₂), or erbiumfluoride (ErF₃)) and a film of a metal such as aluminum. Note that, inthe case in which a charge-generation layer is provided in contact withthe cathode 102 as a part of the second EL layer 107, a variety ofconductive materials can be used for the cathode 102 regardless of themagnitude of their work functions. Note that the charge-generation layercan have the same structure as that of the charge-generation layer 106provided between the first EL layer 103 and the second EL layer 107described above.

Note that in the light-emitting element described in this embodiment, atleast either the anode 101 or the cathode 102 may transmit light havingthe wavelength of the emitted light. The light-transmitting property canbe ensured with use of a transparent electrode such as ITO, or reductionin the thickness of an electrode.

Next, specific examples of the first EL layer and the second EL layerwhich are described above are given.

The first EL layer 103 and the second EL layer 107 each may include atleast a light-emitting layer containing a light-emitting material. Thatis, the first EL layer 103 and the second EL layer 107 may have astructure in which a light-emitting layer and layers other than thelight-emitting layer are stacked. Note that the light-emitting layerincluded in the first EL layer 103 may be different from thelight-emitting layer included in the second EL layer 107. Alternatively,the first EL layer 103 and the second EL layer 107 may independentlyhave a structure in which a light-emitting layer and layers other thanthe light-emitting layer are stacked.

Examples of the layers other than the light-emitting layer include alayer containing a hole-injection material (a hole-injection layer), alayer containing a hole-transport material (a hole-transport layer), alayer containing an electron-transport material (an electron-transportlayer), a layer containing an electron-injection material (anelectron-injection layer), a layer containing a bipolar(electron-transport and hole-transport) material, and the like. Theselayers can be combined as appropriate.

Described below are specific examples of materials contained in layersin the case in which the first EL layer 103 and the second EL layer 107are formed including the hole-injection layer, the hole-transport layer,the light-emitting layer, the electron-transport layer, and theelectron-injection layer.

The hole-injection layer contains a hole-injection material. As thehole-injection material, molybdenum oxide, vanadium oxide, rutheniumoxide, tungsten oxide, manganese oxide, or the like can be used.Alternatively, a phthalocyanine-based compound such as phthalocyanine(abbreviation: H₂Pc) or copper phthalocyanine (abbreviation: CuPc), apolymer such as PEDOT/PSS, or the like can be used as the hole-injectionmaterial.

The hole-transport layer contains a hole-transport material. As thehole-transport material, the following can be given: aromatic aminecompounds such as NPB, TPD, TCTA, TDATA, MTDATA and4,4′-bis[N-(spiro-9,9′-bifluorene-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB); and carbazole derivatives such as PCzPCA1,PCzPCA2, PCzPCN1, CBP, TCPB, and CzPA. Alternatively, it is alsopossible to use the following as the hole-transport material: PVK,PVTPA,poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (abbreviation:Poly-TPD). Most of the substances listed here have a hole mobility ofgreater than or equal to 1×10⁻⁶ cm²/Vs. Note that any other material mayalso be used as long as it is a substance in which the hole-transportproperty is higher than the electron-transport property. Furthermore,the hole-transport layer is not limited to a single layer, but can betwo or more layers formed using the aforementioned materials stacked.

The light-emitting layer contains a light-emitting material. Thelight-emitting material can be a fluorescent compound or aphosphorescent compound which is described below.

Examples of the fluorescent compound include the following:N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-antryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazole-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[i]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM), and the like.

Examples of the phosphorescent compound include the following:bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: FIrpic),bis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIr(acac)),tris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato)iridium(III)acetylacetonato (abbreviation:Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate(abbreviation: Ir(bzq)₂(acac)),bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-perfluorophenylphenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)),bis[2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP),tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)),tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)), and the like.

Note that the light-emitting layer preferably has a structure in whichthese light-emitting materials are dispersed in a host material. As thehost material, the following can be used. For example, it is possible touse a hole-transport material: an aromatic amine compound such as NPB,TPD, TCTA, TDATA, MTDATA, or BSPB; a carbazole derivative such asPCzPCA1, PCzPCA2, PCzPCN1, CBP, TCPB, or CzPA; or a high molecularcompound such as PVK, PVTPA, PTPDMA, or Poly-TPD. It is also possible touse an electron-transport material: a metal complex having a quinolineskeleton or a benzoquinoline skeleton such astris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolate)aluminum (abbreviation:BAlq); a metal complex having an oxazole-based or thiazole-based ligandsuch as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:Zn(BTZ)₂); 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD);1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7);9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]carbazole (abbreviation:CO11); 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen);bathocuproine (abbreviation: BCP);poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation:PF-Py); orpoly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation:PF-BPy).

The electron-transport layer contains an electron-transport material. Asthe electron-transport material, a metal complex having a quinolineskeleton or a benzoquinoline skeleton, such as Alq, Almq₃, BeBq₂, orBAlq can be used. In addition to the above, a metal complex having anoxazole-based or thiazole-based ligand, such as Zn(BOX)₂ or Zn(BTZ)₂ canalso be used. Furthermore, in addition to the above metal complexes,PBD, OXD-7, CO11, TAZ, BPhen, BCP, PF-Py, PF-BPy or the like can also beused as the electron-transport material. Most of the materials listedhere have an electron mobility of greater than or equal to 1×10⁻⁶cm²/Vs. Note that any other material may also be used as long as it is asubstance in which the electron-transport property is higher than thehole-transport property. Furthermore, the electron-transport layer isnot limited to a single layer, but can be two or more layers formedusing the aforementioned materials stacked.

The electron-injection layer contains an electron-injection material. Asthe electron-injection material, the following can be given: an alkalimetal or an alkaline earth metal such as lithium fluoride (LiF), cesiumfluoride (CsF), and calcium fluoride (CaF₂), and a compound thereof.Alternatively, an electron-transport material containing an alkalimetal, an alkaline earth metal, or a compound thereof (e.g., Alq layercontaining magnesium (Mg)) can be used as the electron-injectionmaterial. Such a structure makes it possible to increase the efficiencyof injection of electrons from the cathode 102.

In the case in which a charge-generation layer is provided in the firstEL layer 103 or the second EL layer 107, the charge-generation layercontains a hole-transport material and an acceptor material. Thecharge-generation layer may be not only a layer containing ahole-transport material and an acceptor material in the same film butalso a stack of a layer containing a hole-transport material and a layercontaining an acceptor material. However, in the case of thestacked-layer structure, the layer containing an acceptor material is incontact with the anode 101 or the cathode 102.

The provision of the charge-generation layer in the first EL layer 103or the second EL layer 107 makes it possible to form the anode 101 orthe cathode 102 without consideration of a work function of a materialfor forming the electrodes. Note that the charge-generation layerprovided in the first EL layer 103 or the second EL layer 107 can havethe same structure and can be formed using the same materials as thoseof the charge-generation layer 106 provided between the first EL layer103 and the second EL layer 107 described above. Therefore, the abovedescription is to be referred to.

With the stack of these layers in an appropriate combination, the firstEL layer 103 and the second EL layer 107 can be formed. Further, as aformation method of the first EL layer 103 or the second EL layer 107,any of a variety of methods (e.g., a dry process and a wet process) canbe selected as appropriate in a manner that depends on a material to beused. For example, a vacuum evaporation method, an inkjet method, a spincoating method, or the like may be used. Note that a different formationmethod may be employed for each layer.

The light-emitting element described in this embodiment can bemanufactured by combination of the above-described materials. Becauselight from the above-described light-emitting material in thislight-emitting element can be emitted, a variety of emission colors canbe formed by changing the type of the light-emitting material that isused for the light-emitting layer. In addition, with use of a pluralityof light-emitting materials of different emission colors as thelight-emitting material, light emission having a broad spectrum or whitelight emission can also be performed.

Note that, although the light-emitting element in which two EL layersare provided is described in this embodiment, the number of EL layers isnot limited to two, but may be more than two, for example, three. In thecase in which n EL layers are provided in a light-emitting element (n isa natural number of two or more), with a stack of an electron-injectionbuffer layer, an electron-relay layer, and a charge-generation layerwhich are provided in this order between m-th EL layer from the anodeside and (m+1)-th EL layer (m is a natural number, 1≤m≤n−1), the drivingvoltage of the light-emitting element can be reduced.

Further, the light-emitting element described in this embodiment can bemanufactured over any of a variety of substrates. As the substrate, forexample, a substrate made of glass, plastic, a metal plate, metal foil,or the like can be used. In the case of extracting light emission of thelight-emitting element from the substrate side, a substrate having alight-transmitting property may be used. Note that a substrate otherthan the above may also be used as long as it can serve as a support inthe manufacturing process of the light-emitting element.

Note that the structure in this embodiment can be combined with any ofthe structures in other embodiments as appropriate.

Embodiment 2

In this embodiment, an example of a light-emitting element described inEmbodiment 1 is described. Specifically, described is a case in which anelectron-injection buffer layer 104 included in the light-emittingelement described in Embodiment 1 is formed of a single layer of a donormaterial, with reference to FIGS. 2A and 2B.

In a light-emitting element of this embodiment, as illustrated in FIG.2A, a first EL layer 103 and a second EL layer 107 including alight-emitting region are interposed between a pair of electrodes (ananode 101 and a cathode 102). In addition, the light-emitting elementincludes, from the anode 101 side, the electron-injection buffer layer104 in contact with the first EL layer 103, an electron-relay layer 105in contact with the electron-injection buffer layer 104, and acharge-generation layer 106 in contact with the electron-relay layer 105and the second EL layer 107.

The anode 101, the cathode 102, the first EL layer 103, theelectron-relay layer 105, the charge-generation layer 106, and thesecond EL layer 107 in this embodiment can be formed using materialssimilar to those described in Embodiment 1 and can have structuressimilar to those described in Embodiment 1. Therefore, the descriptionin Embodiment 1 is to be referred to.

In this embodiment, examples of a material used for theelectron-injection buffer layer 104 include the following materialshaving a high electron-injection property: alkali metals such as lithium(Li) and cesium (Cs); alkaline earth metals such as magnesium (Mg),calcium (Ca), and strontium (Sr); rare earth metals such as europium(Eu) and ytterbium (Yb); alkali metal compounds (including an oxide oflithium oxide, a halide, and carbonate such as lithium carbonate andcesium carbonate); alkaline earth metal compounds (including an oxide, ahalide, and carbonate); rare earth metal compounds (including an oxide,a halide, and carbonate); and the like. These materials having a highelectron-injection property are preferred because they are stable in theair, and therefore provide high productivity and are suitable for massproduction.

The light-emitting element described in this embodiment includes asingle layer of the above-described metal or a compound thereof as theelectron-injection buffer layer 104. The thickness of theelectron-injection buffer layer 104 is extremely thin (specifically,less than or equal to 1 nm) in order to avoid an increase in drivingvoltage. Note that in the case in which the electron-injection bufferlayer 104 is formed over the electron-transport layer 108 after theelectron-transport layer 108 is formed, a part of the material used forforming the electron-injection buffer layer 104 can also exist in theelectron-transport layer 108 that is a part of the first EL layer 103.That is, the extremely thin electron-injection buffer layer 104 existsat the interface between the electron-relay layer 105 and theelectron-transport layer 108 that is a part of the first EL layer 103.Note that in this embodiment, the electron-transport layer 108 in thefirst EL layer 103 is preferably formed in contact with theelectron-injection buffer layer 104.

FIG. 2B illustrates a band diagram of the element structure illustratedin FIG. 2A. In FIG. 2B, the electron-injection buffer layer 104 at theinterface between the electron-relay layer 105 and the first EL layer103 (the electron-transport layer 108) can reduce the injection barrierbetween the charge-generation layer 106 and the first EL layer 103 (theelectron-transport layer 108). Thus, electrons generated in thecharge-generation layer 106 can easily be injected to the first EL layer103.

Further, with the structure of the electron-injection buffer layerdescribed in this embodiment, the driving voltage of the light-emittingelement can be reduced more than with a structure of anelectron-injection buffer layer described in Embodiment 3 (a layer thatis formed by adding a donor material to an electron-transport material).

Note that the structure in this embodiment can be combined with any ofthe structures in other embodiments as appropriate.

Embodiment 3

In this embodiment, another example of a light-emitting elementdescribed in Embodiment 1 is described. Specifically, described is acase in which an electron-injection buffer layer 104 included in thelight-emitting element described in Embodiment 1 contains anelectron-transport material and a donor material, with reference toFIGS. 3A and 3B.

In a light-emitting element of this embodiment, as illustrated in FIG.3A, a first EL layer 103 and a second EL layer 107 including alight-emitting region are interposed between a pair of electrodes (ananode 101 and a cathode 102). In addition, the light-emitting elementincludes, from the anode 101 side, the electron-injection buffer layer104 in contact with the first EL layer 103, an electron-relay layer 105in contact with the electron-injection buffer layer 104, and acharge-generation layer 106 in contact with the electron-relay layer 105and the second EL layer 107.

The electron-injection buffer layer 104 contains an electron-transportmaterial and a donor material. Note that in this embodiment, the donormaterial is preferably added so that the weight ratio of the donormaterial to the electron-transport material is from 0.001:1 to 0.1:1, inwhich case the electron-injection buffer layer 104 can have high filmquality and high reactivity.

The anode 101, the cathode 102, the first EL layer 103, theelectron-relay layer 105, the charge-generation layer 106, and thesecond EL layer 107 in this embodiment can be formed using materialssimilar to those described in Embodiment 1 can have structures similarto those described in Embodiment 1. Therefore, the description inEmbodiment 1 is to be referred to.

In this embodiment, examples of the electron-transport materialcontained in the electron-injection buffer layer 104 include thefollowing: a metal complex having a quinoline skeleton or abenzoquinoline skeleton, such as Alq, Almq₃, BeBq₂, and BAlq; a metalcomplex having an oxazole-based or thiazole-based ligand, such asZn(BOX)₂ and Zn(BTZ)₂; PBD; OXD-7; CO11; TAZ; BPhen; BCP; and the like.Most of the substances listed here have an electron mobility of greaterthan or equal to 1×10⁻⁶ cm²/Vs.

In addition to the above substances, a high molecular compound such asPF-Py and PF-BPy can be given as the electron-transport materialcontained in the electron-injection buffer layer 104.

Further, in this embodiment, as the donor material contained in theelectron-injection buffer layer 104, an alkali metal, an alkaline earthmetal, a rare earth metal, a compound thereof (including an oxide, ahalide, and a carbonate, or the like) can be used. Alternatively, anorganic compound such as tetrathianaphthacene (abbreviation: TTN),nickelocene, or decamethylnickelocene can be used as the donor materialcontained in the electron-injection buffer layer 104.

Note that in this embodiment, the electron-transport layer 108 in thefirst EL layer 103 may be formed in contact with the electron-injectionbuffer layer 104. In the case in which the electron-transport layer 108is formed in contact with the electron-injection buffer layer 104, anelectron-transport material used for the electron-injection buffer layer104 and an electron-transport material used for the electron-transportlayer 108 that is a part of the first EL layer 103 may be the same ordifferent.

As illustrated in FIG. 3A, the electron-injection buffer layer 104containing the electron-transport material and the donor material isformed between the first EL layer 103 and the electron-relay layer 105,which is a feature of the light-emitting element described in thisembodiment.

FIG. 3B illustrates a band diagram of the element structure illustratedin FIG. 3A. The electron-injection buffer layer 104 can reduce theinjection barrier between the electron-relay layer 105 and the first ELlayer 103 (the electron-transport layer 108). Thus, electrons generatedin the charge-generation layer 106 can easily be injected to the firstEL layer 103.

Note that the structure in this embodiment can be combined with any ofthe structures in other embodiments as appropriate.

Embodiment 4

In this embodiment, another example of a light-emitting elementdescribed in Embodiment 1 is described. Specifically, described is astructure of a charge-generation layer 106 in the light-emitting elementdescribed in Embodiment 1, with reference to FIGS. 4A and 4B.

In a light-emitting element of this embodiment, as illustrated in FIGS.4A and 4B, a first EL layer 103 and a second EL layer 107 including alight-emitting region are interposed between a pair of electrodes (ananode 101 and a cathode 102). In addition, the light-emitting elementincludes, from the anode 101 side, an electron-injection buffer layer104 in contact with the first EL layer 103, an electron-relay layer 105in contact with the electron-injection buffer layer 104, and thecharge-generation layer 106 in contact with the electron-relay layer 105and the second EL layer 107. In FIGS. 4A and 4B, the anode 101, thecathode 102, the first EL layer 103, the electron-injection buffer layer104, the electron-relay layer 105, and the second EL layer 107 can beformed using materials similar to those described in Embodiments 1 to 3and can have structures similar to those described in Embodiments 1 to3. Therefore, the description in Embodiments 1 to 3 is to be referredto.

In the light-emitting element illustrated in FIGS. 4A and 4B, thecharge-generation layer 106 contains a hole-transport material and anacceptor material. Note that in the charge-generation layer 106,electrons are drawn out from the hole-transport material by the acceptormaterial, whereby holes and electrons are generated.

The charge-generation layer 106 illustrated in FIG. 4A has a structurein which a hole-transport material and an acceptor material arecontained in the same film. In this case, the acceptor material ispreferably added so that the weight ratio of the acceptor material tothe hole-transport material is from 0.1:1 to 4.0:1, in which casecarriers are easily generated in the charge-generation layer 106.

In FIG. 4A, the hole-transport material is doped with the acceptormaterial, and thus an increase in driving voltage can be suppressed evenwhen the thickness of the charge-generation layer 106 is increased. Thesuppression of an increase in driving voltage can lead to improvement ofcolor purity by optical adjustment. In addition, the increase in thethickness of the charge-generation layer 106 can preventshort-circuiting of the light-emitting element.

In contrast, the charge-generation layer 106 illustrated in FIG. 4B hasa structure in which a layer 106 a containing a hole-transport material,which is in contact with the second EL layer 107, and a layer 106 bcontaining an acceptor material, which is in contact with theelectron-relay layer 105, are stacked. In the charge-generation layer106 of the light-emitting element illustrated in FIG. 4B, as a result ofdonation of electrons by the hole-transport material and acceptance ofelectrons by the acceptor material, an electron transfer complex isformed only at the interface between the layer 106 a containing thehole-transport material and the layer 106 b containing the acceptormaterial. Thus, the light-emitting element illustrated in FIG. 4B ispreferable because an absorption band in the visible light region is noteasily formed even when the thickness of the charge-generation layer 106is increased.

The hole-transport material contained in the charge-generation layer 106can be any of a variety of organic compounds such as an aromatic aminecompound, a carbazole derivative, an aromatic hydrocarbon, and a highmolecular compound (e.g., an oligomer, a dendrimer, or a polymer).

Specific examples of the aromatic amine compound include NPB, TPD, TCTA,TDATA, MTDATA, DTDPPA, DPAB, DNTPD, DPA3B, and the like.

Specific examples of the carbazole derivative include PCzPCA1, PCzPCA2,PCzPCN1, and the like. In addition, CBP, TCPB, CzPA,1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and the likecan be given.

Specific examples of the aromatic hydrocarbon include t-BuDNA, DPPA,t-BuDBA, DNA, DPAnth, t-BuAnth, DMNA,9,10-bis[2-(1-naphthyl)phenyl]-2-tert-butylanthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene;tetracene; rubrene, perylene; 2,5,8,11-tetra(tert-butyl)perylene; andthe like. Further, the aromatic hydrocarbon may have a vinyl skeleton.As the aromatic hydrocarbon having a vinyl group, for example, DPVBi,DPVPA, and the like can be given.

Further, a high molecular compound such as PVK or PVTPA can also be usedas the hole-transport material.

The hole-transport material described above preferably has a holemobility of greater than or equal to 1×10⁶ cm²/Vs. Note that any othermaterial may also be used as long as it is a substance in which thehole-transport property is higher than the electron-transport property.

In the case of employing an evaporation method for formation of theabove aromatic hydrocarbon, it is preferable that the number of carbonatoms that forms a condensed ring be 14 to 42 in terms of evaporativityat the time of evaporation or film quality after film formation.

As the acceptor material contained in the charge-generation layer 106,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. Furthermore, as theacceptor material, a transition metal oxide can be given. In addition,oxides of metals belonging to Groups 4 to 8 of the periodic table canalso be given. Specifically, vanadium oxide, niobium oxide, tantalumoxide, chromium oxide, molybdenum oxide, tungsten oxide, manganeseoxide, and rhenium oxide are preferable because their electron-acceptingproperty is high. Furthermore, molybdenum oxide is preferably used asthe acceptor material. Note that molybdenum oxide has a feature of a lowhygroscopic property.

Note that the structure in this embodiment can be combined with any ofthe structures in other embodiments as appropriate.

Embodiment 5

In this embodiment, another example of a light-emitting elementdescribed in Embodiment 1 is described. Specifically, described is anexample of the light-emitting element described in Embodiment 1, withreference to FIGS. 5A and 5B.

In a light-emitting element of this embodiment, as illustrated in FIG.5A, a first EL layer 103 and a second EL layer 107 including alight-emitting region are interposed between a pair of electrodes (ananode 101 and a cathode 102). In addition, the light-emitting elementincludes, from the anode 101 side, an electron-injection buffer layer104 in contact with the first EL layer 103, an electron-relay layer 105in contact with the electron-injection buffer layer 104, and acharge-generation layer 106 in contact with the electron-relay layer 105and the second EL layer 107.

The anode 101, the cathode 102, the electron-injection buffer layer 104,the electron-relay layer 105, and the charge-generation layer 106 inthis embodiment can be formed using materials described in Embodiments 1to 4 and can have structures described in Embodiments 1 to 4. Therefore,the description in Embodiments 1 to 4 is to be referred to.

In this embodiment, the first EL layer 103 includes a firstlight-emitting layer 103 a which exhibits an emission spectrum having apeak in the blue to blue-green wavelength range and a secondlight-emitting layer 103 b which exhibits an emission spectrum having apeak in the yellow to orange wavelength range. Further, the second ELlayer 107 includes a third light-emitting layer 107 a which exhibits anemission spectrum having a peak in the blue-green to green wavelengthrange and a fourth light-emitting layer 107 b which exhibits an emissionspectrum having a peak in the orange to red wavelength range. Note thatthe first light-emitting layer 103 a and the second light-emitting layer103 b may be stacked in reverse order. Note also that the thirdlight-emitting layer 107 a and the fourth light-emitting layer 107 b maybe stacked in reverse order.

When the anode 101 side is positively biased and the cathode 102 side isnegatively biased in such a light-emitting element, holes injected fromthe anode 101 and electrons generated in the charge-generation layer 106and injected through the electron-relay layer 105 and theelectron-injection buffer layer 104 recombine in the firstlight-emitting layer 103 a or the second light-emitting layer 103 b,whereby first light emission 330 is performed. Furthermore, electronsinjected from the cathode 102 and holes generated in thecharge-generation layer 106 and injected recombine in the thirdlight-emitting layer 107 a or the fourth light-emitting layer 107 b,whereby second light emission 340 is performed.

FIG. 5B schematically shows emission spectra of the first light emission330 and the second light emission 340. The first light emission 330 is acombination of light emission from both the first light-emitting layer103 a and the second light-emitting layer 103 b; thus, the first lightemission 330 exhibits an emission spectrum having peaks in both the blueto blue-green wavelength range and the yellow to orange wavelengthrange. That is, the first EL layer 103 exhibits light emission of atwo-wavelength-type white color or a color close to white. Further, thesecond light emission 340 is a combination of light emission from boththe third light-emitting layer 107 a and the fourth light-emitting layer107 b; thus, the second light emission 340 exhibits an emission spectrumhaving peaks in both the blue-green to green wavelength range and theorange to red wavelength range. That is, the second EL layer 107exhibits light emission of two-wavelength-type white color or a colorclose to white, which is different from the light emission of the firstEL layer 103.

As a result of combining the first light emission 330 and the secondlight emission 340, light emission which covers the blue to blue-greenwavelength range, the blue-green to green wavelength range, the yellowto orange wavelength range, and the orange to red wavelength range isobtained with the light-emitting element in this embodiment.

Because the contribution of the first light-emitting layer 103 a withrespect to the entire emission spectrum is approximately one quarter,for example, even in the case in which the luminance of the firstlight-emitting layer 103 a (which exhibits an emission spectrum having apeak in the blue to blue-green wavelength range) deteriorates over timeor changes due to current density, deviation of chromaticity isrelatively small.

Although the example has been described in which the first EL layer 103exhibits the emission spectrum having peaks in both the blue toblue-green wavelength range and the yellow to orange wavelength range,and the second EL layer 107 exhibits the emission spectrum having peaksin both the blue-green to green wavelength range and the orange to redwavelength range, the first EL layer 103 and the second EL layer 107each may exhibit the opposite emission spectrum. In other words, astructure may be employed in which the second EL layer 107 exhibits theemission spectrum having peaks in both the blue to blue-green wavelengthrange and the yellow to orange wavelength range, and the first EL layer103 exhibits the emission spectrum having peaks in both the blue-greento green wavelength range and the orange to red wavelength range.Alternatively, the first EL layer 103 and the second EL layer 107 mayindependently have a structure in which layers other than thelight-emitting layer are stacked.

Next, materials that can be used as a light-emitting organic compoundfor the EL layer of the light-emitting element described in thisembodiment are described. However, materials that can be applied to thelight-emitting element described in this embodiment are not limited tothose given below.

Blue to blue-green light emission can be obtained, for example, by usingperylene, TBP, 9,10-diphenylanthracene, or the like as a guest material,and dispersing the guest material in a suitable host material.Alternatively, the blue to blue-green light emission can be obtainedfrom a styrylarylene derivative such as DPVBi, or an anthracenederivative such as DNA or t-BuDNA. Further alternatively, a polymer suchas poly(9,9-dioctylfluolene) may be used. Further, as a guest materialfor blue light emission, a styrylamine derivative is given such as YGA2SandN,N′-diphenyl-N,N′-bis(9-phenyl-9H-carbazol-3-yl)stilbene-4,4′-diamine(abbreviation: PCA2S). In particular, YGA2S is preferable because it hasa peak near 450 nm. Further, as a host material, an anthracenederivative is preferable; t-BuDNA and CzPA are suitable. In particular,CzPA is preferable because it is electrochemically stable.

Blue-green to green light emission can be obtained, for example, byusing a coumarin dye such as coumarin 30 or coumarin 6; FIrpic;Ir(ppy)₂(acac); or the like as a guest material and dispersing the guestmaterial in a suitable host material. Alternatively, the blue-green togreen light emission can be obtained from a metal complex such as BAlq,Zn(BTZ)₂, or bis(2-methyl-8-quinolinolato)chlorogallium (Ga(mq)₂CL).Further, a polymer such as poly(p-phenylenevinylene) may be used.Alternatively, the blue-green to green light emission can be obtained bydispersing perylene or TBP given above in an appropriate host materialat a high concentration of greater than or equal to 5 wt %. Further, ananthracene derivative is preferably used as a guest material of ablue-green to green light-emitting layer, in which case high emissionefficiency can be obtained. For example, when DPABPA is used, highlyefficient blue-green light emission can be obtained. Further, ananthracene derivative in which an amino group has been substituted intothe 2-position is preferably used, in which case highly efficient greenlight emission can be obtained. In particular, 2PCAPA is suitablebecause it has a long lifetime. As a host material for those materials,an anthracene derivative is preferable; CzPA, which is given above, ispreferable because it is electrochemically stable. Further, in the caseof manufacturing a light-emitting element in which green light emissionand blue light emission are combined and which has two peaks in the blueto green wavelength range, an anthracene derivative having anelectron-transport property, such as CzPA is preferably used as a hostmaterial for a blue light-emitting layer and an aromatic amine compoundhaving a hole-transport property, such as NPB is preferably used as ahost material for a green light-emitting layer, in which case lightemission can be obtained at an interface between the blue light-emittinglayer and the green light-emitting layer. In other words, in such acase, an aromatic amine compound like NPB is preferable as a hostmaterial for a green light-emitting material such as 2PCAPA.

Yellow to orange light emission can be obtained, for example, by usingrubrene, DCM1, DCM2, bis[2-(2-thienyl)pyridinato]acetylacetonatoiridium(abbreviation: Ir(thp)₂(acac)),bis(2-phenylquinolinato)acetylacetonatoiridium (abbreviation:Ir(pq)₂(acac)), or the like as a guest material and dispersing the guestmaterial in a suitable host material. In particular, a tetracenederivative such as rubrene is preferable as a guest material because itis highly efficient and chemically stable. As a host material in thatcase, an aromatic amine compound such as NPB is preferable.Alternatively, a metal complex such as bis(8-quinolinolato)zinc(abbreviation: Znq₂) or bis[2-cinnamoyl-8-quinolinolato]zinc(abbreviation: Znsq₂) can be used as a host material. Furtheralternatively, a polymer such aspoly(2,5-dialkoxy-1,4-phenylenevinylene) may be used.

Orange to red light emission can be obtained, for example, by usingBisDCM, 4-(dicyanomethylene)-2,6-bis[2-(julolidin-9-yl)ethenyl]-4H-pyran(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2), Ir(thp)₂(acac), or the like as a guest materialand dispersing the guest material in a suitable host material.Alternatively, the orange to red light emission can be obtained from ametal complex such as Znq₂ or Znsq₂. Further alternatively, a polymersuch as poly(3-alkylthiophene) may be used. As a guest material whichexhibits red light emission, a 4H-pyran derivative such as BisDCM, DCM2,DCJTI, or BisDCJTM is preferable because it is highly efficient. Inparticular, DCJTI and BisDCJTM are preferable because they have a lightemission peak at approximately 620 nm.

As the appropriate host material in the above-described structures, ahost material which has an emission color of a shorter wavelength thanthe light-emitting organic compound or a host material which has a largeenergy gap may be used. Specifically, a hole-transport material or anelectron-transport material typified by the examples given in Embodiment1 can be selected as appropriate. Alternatively, CBP, TCTA, TCPB, or thelike may be used.

As a result of combining the emission spectrum of the first EL layer andthe emission spectrum of the second EL layer, white light emission whichcovers the blue to blue-green wavelength range, the blue-green to greenwavelength range, the yellow to orange wavelength range, and the orangeto red wavelength range is obtained with the light-emitting elementdescribed in this embodiment.

Note that emitted light may be closer to natural light having acontinuous spectrum in such a manner that slight interference of lightis intentionally caused by adjustment of the film thickness of eachstacked layer so that generation of an extremely sharp peak issuppressed and a trapezoidal emission spectrum is formed. In addition,the slight interference of light which is intentionally generated byadjustment of the film thickness of each stacked layer can also changethe position of a peak of an emission spectrum. By adjusting thethickness of each stacked layer so that a plurality of peak intensitieswhich appear in an emission spectrum are made roughly the same and bydecreasing the intervals between the peaks, white light emission havingan emission spectrum which is closer to a trapezoidal shape can beperformed.

Note that in this embodiment, the EL layer in which the plurallight-emitting layers exhibit complementary emission colors and thesecolors are combined to provide white emission is described. A specificstructure of an EL layer which exhibits white light emission using therelation of complementary colors is described below.

The EL layer provided for the light-emitting element described in thisembodiment has a structure in which, for example, a first layercontaining a hole-transport material and a first light-emittingmaterial; a second layer containing a hole-transport material and asecond light-emitting material; and a third layer containing anelectron-transport material and the second light-emitting material arestacked in this order from an anode 101 side.

Both the first light-emitting material and the second light-emittingmaterial should emit light in order that white light emission isperformed in the EL layers of the light-emitting element described inthis embodiment. For that reason, both the hole-transport material andthe electron-transport material are preferably used as host materials inorder to adjust the transporting properties of carriers in the ELlayers. Note that as the hole-transport material or theelectron-transport material which can be used for the EL layers, thematerials given as examples in Embodiment 1 can be used as appropriate.

Further, as the first light-emitting material and the secondlight-emitting material, materials which emit complementary colors canbe selected and used. Examples of the complementary colors are blue andyellow, blue green and red, and the like. A material which emits blue,yellow, blue-green, or red light may be selected as appropriate from,for example, the light-emitting materials given above. Note that withthe second light-emitting material which has a shorter emissionwavelength than the first light-emitting material, part of excitationenergy of the second light-emitting material is transferred to the firstlight-emitting material, so that the first light-emitting material canemit light. For the above reason, in the light-emitting element of thisembodiment, the emission peak wavelength of the second light-emittingmaterial is preferably shorter than the emission peak wavelength of thefirst light-emitting material.

In the structure of the light-emitting element described in thisembodiment, both light emission from the first light-emitting materialand light emission from the second light-emitting material can beobtained, and the emission color of the first light-emitting materialand the emission color of the second light-emitting material arecomplementary colors, and accordingly white light emission can beobtained. In addition, a light-emitting element with a long lifetime canbe obtained with the structure of the light-emitting element describedin this embodiment.

Note that the structure in this embodiment can be combined with any ofthe structures in other embodiments as appropriate.

Embodiment 6

In this embodiment, modes of light-emitting devices each including thelight-emitting element described in any of Embodiments 1 to 5 aredescribed with reference to FIGS. 6A to 6C. FIGS. 6A to 6C arecross-sectional views of the light-emitting devices.

In FIGS. 6A to 6C, the light-emitting devices in this embodiment includea substrate 10, a light-emitting element 12 and a transistor 11 whichare provided over the substrate 10. The light-emitting element 12includes a layer 15 containing an organic compound between a firstelectrode 13 and a second electrode 14. The layer 15 containing anorganic compound includes n EL layers (n is a natural number of two ormore), and also includes, between m-th EL layer and (m+1)-th EL layer (mis a natural number, 1≤m≤n−1), an electron-injection buffer layer incontact with the m-th EL layer, an electron-relay layer in contact withthe electron-injection buffer layer, and a charge-generation layer incontact with the electron-relay layer and the (m+1)-th EL layer.Further, in a structure of each of the EL layers, at least alight-emitting layer is provided, and in addition to the light-emittinglayer, a hole-injection layer, a hole-transport layer, anelectron-transport layer, or an electron-injection layer is provided asappropriate. That is, the light-emitting element 12 is thelight-emitting element described in any of Embodiments 1 to 5. A drainregion of the transistor 11 is electrically connected to a firstelectrode 13 by a wiring 17 penetrating first interlayer insulatingfilms 16 a, 16 b, and 16 c. The light-emitting element 12 is separatedfrom other adjacently-provided light-emitting elements by partitionlayers 18.

The transistor 11 illustrated in each of FIGS. 6A to 6C is a top-gatetype transistor in which a gate electrode is provided on a side oppositeto the substrate with a semiconductor layer interposed between thesubstrate and the gate electrode. However, there is no particularlimitation on the structure of the transistor 11; for example, thetransistor 11 may be of bottom-gate type. In the case of a bottom-gatetype, the transistor 11 may have a structure in which a protective filmis formed over the semiconductor layer used to form a channel (a channelprotective type) or a structure in which part of the semiconductor layerused to form a channel has a depression (a channel etch type).

The semiconductor layer for forming the transistor 11 may be formedusing any material as long as the material exhibits semiconductorcharacteristics; for example, an element belonging to Group 14 of theperiodic table such as silicon (Si) and germanium (Ge), a compound suchas gallium arsenide and indium phosphide, an oxide such as zinc oxideand tin oxide, and the like can be given. Further, the semiconductorlayer may be either crystalline or non-crystalline.

For the oxide exhibiting semiconductor characteristics (an oxidesemiconductor), composite oxide of an element selected from indium,gallium, aluminum, zinc, and tin can be used. For example, zinc oxide(ZnO), indium oxide containing zinc oxide (IZO: indium zinc oxide), andoxide containing indium oxide, gallium oxide, and zinc oxide (IGZO:indium gallium zinc oxide) can be given. As specific examples of thematerial for the crystalline semiconductor layer, a single crystalsemiconductor, a polycrystalline semiconductor, and a microcrystallinesemiconductor can be given. Such a semiconductor layer may be formed bylaser crystallization or may be formed by crystallization through asolid-phase growth method using, for example, nickel.

Note that the microcrystalline semiconductor in this specificationbelongs to a metastable state which is intermediate between an amorphousstate and a single crystal state when Gibbs free energy is considered.That is, the microcrystalline semiconductor has a third state which isstable in terms of free energy and has a short range order and latticedistortion. The Raman spectrum of microcrystalline silicon, which is atypical example of a microcrystalline semiconductor, is located in lowerwave numbers than 520 cm⁻¹, which represents a peak of the Ramanspectrum of single crystal silicon. That is, the peak of the Ramanspectrum of the microcrystalline silicon exists between 520 cm⁻¹ whichrepresents single crystal silicon and 480 cm⁻¹ which representsamorphous silicon. In addition, microcrystalline silicon containshydrogen or halogen of at least 1 atomic percent or more in order toterminate a dangling bond. Moreover, with addition of a rare gas elementsuch as helium, argon, krypton, or neon in order to further promotelattice distortion, stability is enhanced and a favorablemicrocrystalline semiconductor layer can be formed.

In the case in which the semiconductor layer is formed using anamorphous material, for example, amorphous silicon, it is preferablethat the light-emitting device have a circuit in which the transistor 11and other transistors (transistors constituting a circuit for drivingthe light-emitting element) are all n-channel transistors because themanufacturing process of the light-emitting device is simplified.Further, zinc oxide (ZnO), indium oxide containing zinc oxide (IZO),oxide containing indium oxide, gallium oxide, and zinc oxide (IGZO), andthe like are n-type semiconductors; thus, a transistor in which any ofthose oxides is contained in a semiconductor layer is of n-channel type.The light-emitting device may have a circuit including either ann-channel transistor or a p-channel transistor, or may have a circuitincluding both an n-channel transistor and a p-channel transistor.

The first interlayer insulating films 16 a to 16 c may have a multilayerstructure as illustrated in FIGS. 6A and 6C, or may have a single layerstructure. Note that the first interlayer insulating film 16 a is formedof an inorganic material such as silicon oxide or silicon nitride; thefirst interlayer insulating film 16 b is formed of acrylic, siloxane (anorganic group including a skeleton structure of a bond of silicon (Si)and oxygen (O) and containing at least hydrogen as a substituent) or aself-planarizing material which can be formed as a film by anapplication method, such as silicon oxide. In addition, the firstinterlayer insulating film 16 c is formed of a silicon nitride filmcontaining argon (Ar). Note that there is no particular limitation onthe material forming each layer, and a material other than the abovematerials may also be used. A layer formed using a material other thanthe above materials may be further combined. As described above, thefirst interlayer insulating films 16 a to 16 c may be formed usingeither an inorganic material or an organic material, or both of them.

As for the partition layer 18, the radius of curvature of the edgeportion preferably changes continuously. In addition, the partitionlayer 18 can be formed using acrylic, siloxane, silicon oxide, or thelike. Note that the partition layer 18 may be formed using either aninorganic material or an organic material, or both of them.

Although the structure in which only the first interlayer insulatingfilms 16 a to 16 c are provided between the transistor 11 and thelight-emitting element 12 is illustrated in each of FIGS. 6A and 6C, inaddition to the first interlayer insulating films 16 a to 16 c, secondinterlayer insulating films 19 a and 19 b may be provided as illustratedin FIG. 6B. In the light-emitting device illustrated in FIG. 6B, thefirst electrode 13 penetrates the second interlayer insulating films 19a and 19 b to be electrically connected to the wiring 17.

The second interlayer insulating films 19 a and 19 b may have amultilayer structure or may have a single layer structure in a mannersimilar to that of the first interlayer insulating films 16 a to 16 c.The second interlayer insulating film 19 a is formed of acrylic,siloxane, or a self-planarizing material which can be formed as a filmby an application method, such as silicon oxide. The second interlayerinsulating film 19 b is formed of a silicon nitride film containingargon (Ar). Note that there is no particular limitation on the materialforming each layer, and a material other than the above materials mayalso be used. A layer formed of a material other than the abovematerials may be further combined. As described above, the secondinterlayer insulating films 19 a and 19 b may be formed using either aninorganic material or an organic material, or both of them.

In the case in which both the first electrode 13 and the secondelectrode 14 in the light-emitting element 12 are formed using alight-transmitting material, emitted light can be extracted from boththe first electrode 13 and the second electrode 14 as indicated by theoutline arrows in FIG. 6A. In addition, in the case in which only thesecond electrode 14 is formed using a light-transmitting material,emitted light can be extracted from only the second electrode 14 asindicated by the outline arrow in FIG. 6B. In that case, it ispreferable that the first electrode 13 be formed using a material havinghigh reflectivity, or that a film formed using a material having highreflectivity (a reflective film) be provided under the first electrode13. Furthermore, in the case in which only the first electrode 13 isformed using a light-transmitting material, emitted light can beextracted from only the first electrode 13 as indicated by the outlinearrow in FIG. 6C. In that case, it is preferable that the secondelectrode 14 be formed using a material having high reflectivity, orthat a reflective film be formed above the second electrode 14.

Further, in the light-emitting element 12, the layer 15 containing anorganic compound may be stacked so that the light-emitting element 12operates when a voltage is applied such that the potential of the secondelectrode 14 becomes higher than that of the first electrode 13, or suchthat the potential of the second electrode 14 becomes lower than that ofthe first electrode 13. In the former case, the first electrode 13 is ananode, the second electrode 14 is a cathode, and the transistor 11 is ann-channel transistor; in the latter case, the first electrode 13 is acathode, the second electrode 14 is an anode, and the transistor 11 is ap-channel transistor.

In this embodiment, an active-matrix light-emitting device in whichdriving of a light-emitting element is controlled by a transistor isdescribed. In addition, a passive-matrix light-emitting device in whicha light-emitting element is driven without provision of an element fordriving a transistor or the like on the substrate over which thelight-emitting element is formed may be manufactured. FIG. 7A is aperspective view of a passive-matrix light-emitting device manufacturedby application of the light-emitting element described in any ofEmbodiments 1 to 5. In addition, FIG. 7B is a cross-sectional view takenalong dashed line X-Y of FIG. 7A.

In FIGS. 7A and 7B, a light-emitting element 955 is provided between anelectrode 952 and an electrode 956 over a substrate 951. Thelight-emitting element 955 is the light-emitting element described inany of Embodiments 1 to 5. End portions of the electrode 952 are coveredwith an insulating layer 953. Then, a partition layer 954 is providedover the insulating layer 953. The partition layer 954 preferably hastapered side surfaces with such a slope that the distance betweenopposite side surfaces decreases toward the substrate surface. That is,a cross section of the partition layer 954 in the direction of a narrowside is trapezoidal, and a base (a side facing in a direction similar toa plane direction of the insulating layer 953 and being in contact withthe insulating layer 953) is shorter than an upper side (a side facingin a direction similar to the plane direction of the insulating layer953 and not being in contact with the insulating layer 953). Thepartition layer 954 as described above can prevent a defect of thelight-emitting element due to static electricity or the like. Thepassive-matrix light-emitting device can also be driven with low powerconsumption when it includes the light-emitting element described in anyof Embodiments 1 to 5.

The light-emitting element described as an example in any of Embodiments1 to 5 is used in the light-emitting device described in thisembodiment; thus, the light-emitting device can have a high luminance,can be driven at a low voltage, and consumes less power.

Embodiment 7

In this embodiment, described are electronic devices including thelight-emitting devices examples of which are described in Embodiment 6.

As examples of the electronic devices of this embodiment, the followingcan be given: televisions, cameras such as video cameras and digitalcameras, goggle type displays, navigation systems, computers, gamemachines, portable information terminals (e.g., mobile computers,cellular phones, portable game machines, and electronic book readers),image replay devices in which a recording medium is provided(specifically, devices that are capable of replaying recording mediasuch as digital versatile discs (DVDs) and equipped with a displayportion that can display an image), and the like. Specific examples ofthese electronic devices are illustrated in FIGS. 8A to 8E.

FIG. 8A illustrates an example of a portable information terminal device800. The portable information terminal device 800 incorporates acomputer and therefore can process a variety of types of data. As anexample of the portable information terminal device 800, a personaldigital assistant (PDA) can be given.

The portable information terminal device 800 has two housings: a housing801 and a housing 803. The housing 801 and the housing 803 are joinedwith a joining portion 807 such that the portable information terminaldevice 800 can be foldable. A display portion 802 is incorporated in thehousing 801, and the housing 803 is provided with a keyboard 805.Needless to say, the structure of the portable information terminaldevice 800 is not limited to the one described above, and the portableinformation terminal device 800 may be provided with other accessoriesas appropriate. In the display portion 802, light-emitting elementssimilar to those described in any of Embodiments 1 to 5 are arranged inmatrix. The light-emitting elements have features of a high luminance,low driving voltage, and low power consumption. The display portion 802including those light-emitting elements has similar features; therefore,low power consumption of this portable information terminal device canbe achieved.

FIG. 8B illustrates an example of a digital video camera 810 of thisembodiment. The digital video camera 810 includes a display portion 812incorporated in a housing 811 and various operation portions. Note thatthere is no particular limitation on the structure of the digital videocamera 810, and the digital video camera 810 may be provided with otheraccessories as appropriate.

In this digital video camera 810, the display portion 812 includeslight-emitting elements similar to those described in any of Embodiments1 to 5, which are arranged in matrix. The light-emitting elements havefeatures of low driving voltage, a high luminance, and low powerconsumption. The display portion 812 including those light-emittingelements has similar features; therefore, low power consumption of thisdigital video camera 810 can be achieved.

FIG. 8C illustrates an example of a cellular phone 820 of thisembodiment. The cellular phone 820 has two housings: a housing 821 and ahousing 822. The housing 821 and the housing 822 are joined with ajoining portion 823 such that the cellular phone can be foldable. Adisplay portion 824 is incorporated in the housing 822, and the housing821 is provided with operation keys 825. Note that there is noparticular limitation on the structure of the cellular phone 820, andthe cellular phone 820 may be provided with other accessories asappropriate.

In the cellular phone 820, the display portion 824 includeslight-emitting elements similar to those described in any of Embodiments1 to 5, which are arranged in matrix. The light-emitting elements havefeatures of a high luminance, low driving voltage, and low powerconsumption. The display portion 824 including those light-emittingelements has similar features; therefore, low power consumption of thiscellular phone can be achieved. As a backlight of a display provided fora cellular phone or the like, the light-emitting element described inany of the above embodiments may be used.

FIG. 8D illustrates an example of a portable computer 830. The computer830 has two housings: a housing 831 and a housing 834 that are joinedsuch that the computer 830 can be opened and closed. A display portion832 is incorporated in the housing 831, and the housing 834 is providedwith a keyboard 833 and the like. Note that there is no particularlimitation on the structure of the computer 830, and the computer 830may be provided with other accessories as appropriate.

In this computer 830, the display portion 832 includes light-emittingelements similar to those described in the any of Embodiments 1 to 5,which are arranged in matrix. The light-emitting elements have featuresof a high luminance, low driving voltage, and low power consumption. Thedisplay portion 832 including those light-emitting elements has similarfeatures; therefore, low power consumption of this computer can beachieved.

FIG. 8E illustrates an example of a television set 840. In thetelevision set 840, a display portion 842 is incorporated in a housing841. The display portion 842 can display images. Here, the housing 841is supported by a stand 843.

The television set 840 can be operated with an operation switch (notillustrated) of the housing 841 or a separate remote controller 850.Channels can be selected and volume can be controlled with an operationkey 851 of the remote controller 850, so that images displayed on thedisplay portion 842 can be controlled. Furthermore, the remotecontroller 850 may be provided with a display portion 852 for displayinginformation outputted from the remote controller 850.

Note that the television set 840 is provided with a receiver, a modem,and the like. With the use of the receiver, a general televisionbroadcast can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

In at least either the display portion 842 or the display portion 852 ofthe television set 840, light-emitting elements similar to thosedescribed in any of Embodiments 1 to 5 are arranged in matrix. Thelight-emitting elements have features of a high luminance, low drivingvoltage, and low power consumption. The display portion including thoselight-emitting elements also has similar features.

As described above, the application range of the light-emitting deviceis so wide that this light-emitting device can be applied to electronicdevices in all fields. With use of the light-emitting devices includingthe light-emitting elements described in Embodiments 1 to 5, anelectronic device having a low-power-consumption display portion whichexhibits high luminance light emission can be provided.

Embodiment 8

In this embodiment, described are lighting devices including thelight-emitting device examples of which are described in Embodiment 6.

FIG. 9 illustrates indoor lighting devices. A desk lighting device 900in FIG. 9 includes a lighting portion 901. In the lighting portion 901,the light-emitting elements described in any of Embodiments 1 to 5 areused. The light-emitting elements have features of a high luminance, lowdriving voltage, and low power consumption. The lighting portion 901including those light-emitting elements has similar features; thus, lowpower consumption of the desk lighting device 900 can be achieved. Sincethis light-emitting element can have a larger area, the light-emittingelement can be applied to a lighting portion 911 of a ceiling lightingdevice 910. Moreover, this light-emitting element can be flexible andtherefore can be applied to a lighting portion 921 of a roll-typelighting device 920.

FIG. 10A illustrates a traffic light. A traffic light 1000 includes agreen lighting portion 1001, an amber lighting portion 1002, and a redlighting portion 1003. The traffic light 1000 includes thelight-emitting element described in any of Embodiments 1 to 5 in atleast one of the lighting portions corresponding to green, amber, andred. The light-emitting elements have features of a high luminance, lowdriving voltage, and low power consumption. The lighting portionscorresponding to green, amber, or red which includes thoselight-emitting elements have similar features; thus, low powerconsumption of this traffic light can be achieved.

FIG. 10B illustrates an emergency exit light. An emergency exit light1010 can be formed with combination of a lighting portion and afluorescent plate provided with a fluorescent portion. The emergencyexit light 1010 can also be formed with combination of a lightingportion emitting a specific light and a light-shielding plate providedwith a transmitting portion having a shape illustrated in FIG. 10B. Inthe lighting portion of the emergency exit light 1010, thelight-emitting elements described in any of Embodiments 1 to 5 are used.The light-emitting elements have features of a high luminance, lowdriving voltage, and low power consumption. The lighting portionincluding those light-emitting elements has similar features; therefore,low power consumption of this emergency exit light can be achieved.

FIG. 10C illustrates a streetlight. The streetlight includes a support1021 and a lighting portion 1022. In the lighting portion 1022, thelight-emitting elements described in any of Embodiments 1 to 5 are used.The light-emitting elements have features of a high luminance, lowdriving voltage, and low power consumption. The lighting portionincluding those light-emitting elements has similar features; therefore,low power consumption of this streetlight can be achieved.

Note that a power source voltage can be supplied to the streetlightthrough a power line 1024 on a utility pole 1023 as illustrated in FIG.10C. Note that the method for supplying the power source voltage is notlimited to this case; for example, a photoelectric converter may beprovided in the support 1021 so that voltage obtained from thephotoelectric converter can be used as a power source voltage.

FIGS. 10D and 10E illustrate examples in which a lighting device isapplied to a portable light. FIG. 10D illustrates a structure of awearable light and FIG. 10E illustrates a structure of a handheld light.

FIG. 10D illustrates a wearable light. The wearable light includes amounting portion 1031 and a lighting portion 1032 fixed to the mountingportion 1031. In the lighting portion 1032, the light-emitting elementsdescribed in any of Embodiments 1 to 5 are used. The light-emittingelements have features of a high luminance, low driving voltage, and lowpower consumption. The lighting portion including those light-emittingelements has similar features; therefore, low power consumption of thiswearable light can be achieved.

Note that the structure of the wearable light is not limited to thatillustrated in FIG. 10D, and for example, the following structure can beemployed: the mounting portion 1031 is formed as a ring belt of flatbraid or elastic braid, the lighting portion 1032 is fixed to the belt,and the belt is directly tied around the head.

FIG. 10E illustrates a handheld light. The handheld light includes ahousing 1041, a lighting portion 1042, and a switch 1043. In thelighting portion 1042, the light-emitting elements described in any ofEmbodiments 1 to 5 are used. The light-emitting elements have featuresof a high luminance, low driving voltage, and low power consumption. Thelighting portion 1042 including those light-emitting elements hassimilar features; thus, low power consumption of this handheld light canbe achieved.

The switch 1043 has a function of controlling emission or non-emissionof the lighting portion 1042. The switch 1043 can also have a functionof controlling, for example, the luminance of the lighting portion 1042during light emission.

As described above, the application range of the light-emitting deviceis so wide that the light-emitting device can be applied to lightingdevices in all fields. With use of a lighting device including thelight-emitting elements described in Embodiments 1 to 5, a lightingdevice having a low-power-consumption display portion which exhibitshigh luminance light emission can be provided.

Example 1

In this example, light-emitting elements that are one embodiment of thepresent invention are described with reference to FIGS. 11A and 11B.Chemical formulae of the materials used in this example and Examples 2and 3 are shown below.

Methods for manufacturing light-emitting elements 1 to 4 and a referencelight-emitting element 5 in this example are described below.

First, the light-emitting element 1 is described (see FIG. 11A). Indiumtin oxide containing silicon oxide was deposited over a substrate 2100by a sputtering method to form a first electrode 2101. The firstelectrode 2101 had a thickness of 110 nm and an area of 2 mm×2 mm.

Next, the substrate 2100 on which the first electrode 2101 was formedwas fixed to a substrate holder provided in a vacuum evaporationapparatus in such a way that a surface of the substrate on which thefirst electrode 2101 was formed faced downward, and then the pressurewas reduced to approximately 10⁻⁴ Pa. After that, NPB that is ahole-transport material and molybdenum(VI) oxide that is an acceptormaterial were co-evaporated on the first electrode 2101 to form a firstcharge-generation layer 2103 a containing a composite material of anorganic compound and an inorganic compound. The thickness of the firstcharge-generation layer 2103 a was 50 nm. The weight ratio of NPB tomolybdenum(VI) oxide was adjusted to 4:1 (=NPB:molybdenum oxide). Notethat the co-evaporation method is an evaporation method in whichevaporation is carried out using a plurality of evaporation sourcessimultaneously in one treatment chamber.

Next, NPB was deposited to a thickness of 10 nm on the firstcharge-generation layer 2103 a by an evaporation method using resistanceheating to form a hole-transport layer 2103 b.

Furthermore, CzPA and 2PCAPA were co-evaporated to form a light-emittinglayer 2103 c with a thickness of 30 nm on the hole-transport layer 2103b. Here, the weight ratio of CzPA to 2PCAPA was adjusted to 1:0.05(=CzPA:2PCAPA). Note that CzPA is an electron-transport material and2PCAPA that is a guest material is a material exhibiting green lightemission.

After that, Alq was deposited on the light-emitting layer 2103 c to athickness of 10 nm by an evaporation method using resistance heating toform an electron-transport layer 2103 d. Thus, a first EL layer 2103including the first charge-generation layer 2103 a, the hole-transportlayer 2103 b, the light-emitting layer 2103 c, and theelectron-transport layer 2103 d was formed.

Next, BPhen and lithium (Li) were co-evaporated to form anelectron-injection buffer layer 2104 with a thickness of 20 nm on theelectron-transport layer 2103 d. Here, the weight ratio of BPhen tolithium (Li) was adjusted to 1:0.02 (=BPhen:Li).

Next, PTCBI and lithium (Li) were co-evaporated to form anelectron-relay layer 2105 with a thickness of 3 nm on theelectron-injection buffer layer 2104. Here, the weight ratio of PTCBI tolithium (Li) was adjusted to 1:0.02 (=PTCBI:Li). Note that the LUMOlevel of PTCBI is approximately −4.0 eV according to the result ofcyclic voltammetry (CV).

Next, NBP that is a hole-transport material and molybdenum(VI) oxidethat is an acceptor material were co-evaporated on the electron-relaylayer 2105 to form a second charge-generation layer 2106. The thicknessof the second charge-generation layer 2106 was 60 nm. The weight ratioof NPB to molybdenum(VI) oxide was adjusted to 4:1 (=NPB:molybdenumoxide).

Next, a second EL layer 2107 was formed on the second charge-generationlayer 2106. A method for manufacturing the second EL layer 2107 isdescribed below. First, NPB was deposited to a thickness of 10 nm on thesecond charge-generation layer 2106 to form a hole-transport layer 2107a by an evaporation method using resistance heating.

After that, CzPA and 2PCAPA were co-evaporated to form a light-emittinglayer 2107 b with a thickness of 30 nm on the hole-transport layer 2107a. Here, the weight ratio of CzPA to 2PCAPA was adjusted to 1:0.05(=CzPA:2PCAPA). That is, the structure of the light-emitting layer 2107b included in the second EL layer 2107 was the same as that of thelight-emitting layer 2103 c included in the first EL layer 2103.

Next, Alq with a thickness of 10 nm and BPhen with a thickness of 20 nmwere stacked on the light-emitting layer 2107 b by evaporation to forman electron-transport layer 2107 c. Then, lithium fluoride (LiF) wasevaporated to a thickness of 1 nm on the electron-transport layer 2107 cto form an electron-injection layer 2107 d. Thus, the second EL layer2107 including the hole-transport layer 2107 a, the light-emitting layer2107 b, the electron-transport layer 2107 c, and the electron-injectionlayer 2107 d was formed.

Lastly, aluminum (Al) was deposited to a thickness of 200 nm on theelectron-injection layer 2107 d by an evaporation method usingresistance heating to form a second electrode 2102. Thus, thelight-emitting element 1 was manufactured.

Next, the light-emitting element 2 is described. The light-emittingelement 2 was manufactured in a manner similar to that of thelight-emitting element 1, except for an electron-relay layer 2105.Therefore, for the structure and the manufacturing method of thelight-emitting element 2, the description above is to be referred to,except for the electron-relay layer 2105. As the electron-relay layer2105 included in the light-emitting element 2, PPDN, which is anelectron-transport material and lithium (Li), which is a donor materialwere co-evaporated to a thickness of 3 nm on the electron-injectionbuffer layer 2104. Here, the weight ratio of PPDN to lithium (Li) wasadjusted to 1:0.02 (=PPDN:Li). Note that the LUMO level of PPDN isapproximately −3.83 eV according to the result of the measurements bycyclic voltammetry (CV).

Next, the light-emitting element 3 is described. The light-emittingelement 3 was manufactured in a manner similar to that of thelight-emitting element 1, except for an electron-relay layer 2105.Therefore, for the structure and the manufacturing method of thelight-emitting element 3, the description above is to be referred to,except for the electron-relay layer 2105. As the electron-relay layer2105 included in the light-emitting element 3, PTCBI, which is anelectron-transport material and lithium oxide (Li₂O), which is a donormaterial were co-evaporated to a thickness of 3 nm on theelectron-injection buffer layer 2104. Here, the weight ratio of PTCBI tolithium oxide (Li₂O) was adjusted to 1:0.02 (=PTCBI:Li₂O).

Next, the light-emitting element 4 is described. The light-emittingelement 4 was manufactured in a manner similar to that of thelight-emitting element 1, except for an electron-relay layer 2105.Therefore, for the structure and the manufacturing method of thelight-emitting element 4, the description above is to be referred to,except for the electron-relay layer 2105. As the electron-relay layer2105 included in the light-emitting element 4, PPDN, which is anelectron-transport material and lithium oxide (Li₂O), which is a donormaterial were co-evaporated to a thickness of 3 nm on theelectron-injection buffer layer 2104. Here, the weight ratio of PPDN tolithium oxide (Li₂O) was adjusted to 1:0.02 (=PPDN:Li₂O).

Next, the reference light-emitting element 5 is described (see FIG.11B). The reference light-emitting element 5 had the structure of thelight-emitting elements 1 to 4, from which the electron-relay layer 2105was removed. The other layers were formed by manufacturing methodssimilar to those of the light-emitting elements 1 to 4. That is, in thereference light-emitting element 5, after the electron-injection bufferlayer 2104 was formed, the second charge-generation layer 2106 wasformed on the electron-injection buffer layer 2104. Thus, the referencelight-emitting element 5 of this example was formed.

Table 1 below shows the element structures of the light-emittingelements 1 to 4 and the reference light-emitting element 5.

TABLE 1 Light-emitting Light-emitting Light-emitting Light-emittingReference Light- Element 1 Element 2 Element 3 Element 4 emittingElement 5 2101 ITSO ITSO ITSO ITSO ITSO 110 nm 110 nm 110 nm 110 nm 110nm 2103 2103a NPB:MoOx NPB:MoOx NPB:MoOx NPB:MoOx NPB:MoOx (=4:1) (=4:1)(=4:1) (=4:1) (=4:1)  50 nm  50 nm  50 nm  50 nm  50 nm 2103b NPB NPBNPB NPB NPB  10 nm  10 nm  10 nm  10 nm  10 nm 2103c CzPA:2PCAPACzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPA (=1:0.05) (=1:0.05)(=1:0.05) (=1:0.05) (=1:0.05)  30 nm  30 nm  30 nm  30 nm  30 nm 2103dAlq Alq Alq Alq Alq  10 nm  10 nm  10 nm  10 nm  1 nm 2104 BPhen:LiBPhen:Li BPhen:Li BPhen:Li BPhen:Li (=1:0.02) (=1:0.02) (=1:0.02)(=1:0.02) (=1:0.02)  20 nm  20 nm  20 nm  20 nm  20 nm 2105 PTCBI:LiPPDN:Li PTCBI:Li2O PPDN:Li2O — (=1:0.02) (=1:0.02) (=1:0.02) (=1:0.02) 3 nm  3 nm  3 nm  3 nm 2106 NPB:MoOx NPB:MoOx NPB:MoOx NPB:MoOxNPB:MoOx (=4:1) (=4:1) (=4:1) (=4:1) (=4:1)  60 nm  60 nm  60 nm  60 nm 60 nm 2107 2107a NPB NPB NPB NPB NPB  10 nm  10 nm  10 nm  10 nm  10 nm2107b CzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPA(=1:0.05) (=1:0.05) (=1:0.05) (=1:0.05) (=1:0.05)  30 nm  30 nm  30 nm 30 nm  30 nm 2107c Alq Alq Alq Alq Alq  10 nm  10 nm  10 nm  10 nm  10nm BPhen BPhen BPhen BPhen BPhen  20 nm  20 nm  20 nm  20 nm  20 nm2107d LiF LiF LiF LiF LiF  1 nm  1 nm  1 nm  1 nm  1 nm 2102 Al Al Al AlAl 200 nm 200 nm 200 nm 200 nm 200 nm

The thus obtained light-emitting elements 1 to 4 and referencelight-emitting element 5 were sealed in a glove box under a nitrogenatmosphere so that they were not exposed to atmospheric air. After that,the operating characteristics of these light-emitting elements weremeasured. The measurement was carried out at room temperature (under anatmosphere in which the temperature was kept at 25° C.).

FIG. 12 shows voltage-luminance characteristics of the light-emittingelements 1 to 4 and the reference light-emitting element 5. In FIG. 12,the horizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 13 shows voltage-currentdensity characteristics of the light-emitting elements 1 to 4 and thereference light-emitting element 5. In FIG. 13, the horizontal axisrepresents voltage (V) and the vertical axis represents current density(mA/cm²). Moreover, Table 2 below shows voltages of the light-emittingelements at a luminance of approximately 1000 cd/m².

TABLE 2 Voltage (V) Light-emitting Element 1 7 4 Light-emitting Element2 7.4 Light-emitting Element 3 7.4 Light-emitting Element 4 7.4Reference Light-emitting Element 5 8.0

FIG. 12 indicates that the light-emitting elements 1 to 4 in each ofwhich the electron-relay layer is provided have a higher luminance thanthe reference light-emitting element 5. In addition, FIG. 13 indicatesthat the light-emitting elements 1 to 4 in each of which theelectron-relay layer is provided have a higher current density than thereference light-emitting element 5.

Thus, it was confirmed that the light-emitting elements 1 to 4 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting elements 1to 4 were light-emitting elements capable of being driven at a lowervoltage than the reference light-emitting element 5.

Example 2

In this example, light-emitting elements that are one embodiment of thepresent invention are described with reference to FIGS. 11A and 11B.Note that in light-emitting elements and a reference light-emittingelement in this example, for the part that is the same as or similar tothat described in Example 1, the description above is to be referred to.

Methods for manufacturing light-emitting elements 6 to 9 and a referencelight-emitting element 10 of this example are described below.

First, the light-emitting element 6 is described (see FIG. 11A). Thelight-emitting element 6 of this example was manufactured in a mannersimilar to that of the light-emitting element 1 described in Example 1,except for the electron-injection buffer layer 2104. Therefore, for thestructure and the manufacturing method of the light-emitting element 6,the description in Example 1 is to be referred to, except for theelectron-injection buffer layer 2104. In the light-emitting element 6 ofthis example, BPhen, which is an electron-transport material and lithiumoxide (Li₂O), which is a donor material were co-evaporated to form theelectron-injection buffer layer 2104 to a thickness of 20 nm on theelectron-transport layer 2103 d. Here, the weight ratio of BPhen tolithium oxide (Li₂O) was adjusted to 1:0.02 (=BPhen:Li₂O).

Next, the light-emitting element 7 is described. The light-emittingelement 7 of this example was manufactured in a manner similar to thatof the light-emitting element 2 described in Example 1, except for theelectron-injection buffer layer 2104. Therefore, for the structure andthe manufacturing method of the light-emitting element 7, thedescription in Example 1 is to be referred to, except for theelectron-injection buffer layer 2104. In the light-emitting element 7 ofthis example, BPhen, which is an electron-transport material and lithiumoxide (Li₂O), which is a donor material were co-evaporated to form theelectron-injection buffer layer 2104 to a thickness of 20 nm on theelectron-transport layer 2103 d. Here, the weight ratio of BPhen tolithium oxide (Li₂O) was adjusted to 1:0.02 (=BPhen:Li₂O).

Next, the light-emitting element 8 is described. The light-emittingelement 8 of this example was manufactured in a manner similar to thatof the light-emitting element 3 described in Example 1, except for theelectron-injection buffer layer 2104. Therefore, for the structure andthe manufacturing method of the light-emitting element 8, thedescription in Example 1 is to be referred to, except for theelectron-injection buffer layer 2104. In the light-emitting element 8 ofthis example, BPhen, which is an electron-transport material and lithiumoxide (Li₂O), which is a donor material were co-evaporated to form theelectron-injection buffer layer 2104 to a thickness of 20 nm on theelectron-transport layer 2103 d. Here, the weight ratio of BPhen tolithium oxide (Li₂O) was adjusted to 1:0.02 (=BPhen:Li₂O).

Next, the light-emitting element 9 is described. The light-emittingelement 9 of this example was manufactured in a manner similar to thatof the light-emitting element 4 described in Example 1, except for theelectron-injection buffer layer 2104. Therefore, for the structure andthe manufacturing method of the light-emitting element 9, thedescription in Example 1 is to be referred to, except for theelectron-injection buffer layer 2104. In the light-emitting element 9 ofthis example, BPhen, which is an electron-transport material and lithiumoxide (Li₂O), which is a donor material were co-evaporated to form theelectron-injection buffer layer 2104 to a thickness of 20 nm on theelectron-transport layer 2103 d. Here, the weight ratio of BPhen tolithium oxide (Li₂O) was adjusted to 1:0.02 (=BPhen:Li₂O).

Next, the reference light-emitting element 10 is described (see FIG.11B). The reference light-emitting element 10 had the structure of thelight-emitting elements 6 to 9, from which the electron-relay layer 2105was removed. The other layers were formed by manufacturing methodssimilar to those of the light-emitting elements 6 to 9. That is, in thereference light-emitting element 10, after the electron-injection bufferlayer 2104 was formed, the second charge-generation layer 2106 wasformed on the electron-injection buffer layer 2104. Thus, the referencelight-emitting element 10 of this example was obtained.

Table 3 below shows the element structures of the light-emittingelements 6 to 9 and the reference light-emitting element 10.

TABLE 3 Light-emitting Light-emitting Light-emitting Light-emittingReference Light- Element 6 Element 7 Element 8 Element 9 emittingElement 10 2101 ITSO ITSO ITSO ITSO ITSO 110 nm 110 nm 110 nm 110 nm 110nm 2103 2103a NPB:MoOx NPB:MoOx NPB:MoOx NPB:MoOx NPB:MoOx (=4:1) (=4:1)(=4:1) (=4:1) (=4:1)  50 nm  50 nm  50 nm  50 nm  50 nm 2103b NPB NPBNPB NPB NPB  10 nm  10 nm  10 nm  10 nm  10 nm 2103c CzPA:2PCAPACzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPA (=1:0.05) (=1:0.05)(=1:0.05) (=1:0.05) (=1:0.05)  30 nm  30 nm  30 nm  30 nm  30 nm 2103dAlq Alq Alq Alq Alq  10 nm  10 nm  10 nm  10 nm  1 nm 2104 BPhen:Li₂OBPhen:Li₂O BPhen:Li₂O BPhen:Li₂O BPhen:Li₂O (=1:0.02) (=1:0.02)(=1:0.02) (=1:0.02) (=1:0.02)  20 nm  20 nm  20 nm  20 nm  20 nm 2105PTCBI:Li PPDN:Li PTCBI:Li₂O PPDN:Li₂O — (=1:0.02) (=1:0.02) (=1:0.02)(=1:0.02)  3 nm  3 nm  3 nm  3 nm 2106 NPB:MoOx NPB:MoOx NPB:MoOxNPB:MoOx NPB:MoOx (=4:1) (=4:1) (=4:1) (=4:1) (=4:1)  60 nm  60 nm  60nm  60 nm  60 nm 2107 2107a NPB NPB NPB NPB NPB  10 nm  10 nm  10 nm  10nm  10 nm 2107b CzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPA CzPA:2PCAPACzPA:2PCAPA (=1:0.05) (=1:0.05) (=1:0.05) (=1:0.05) (=1:0.05)  30 nm  30nm  30 nm  30 nm  30 nm 2107c Alq Alq Alq Alq Alq  10 nm  10 nm  10 nm 10 nm  10 nm BPhen BPhen BPhen BPhen BPhen  20 nm  20 nm  20 nm  20 nm 20 nm 2107d LiF LiF LiF LiF LiF  1 nm  1 nm  1 nm  1 nm  1 nm 2102 AlAl Al Al Al 200 nm 200 nm 200 nm 200 nm 200 nm

The thus obtained light-emitting elements 6 to 9 and referencelight-emitting element 10 were sealed in a glove box under a nitrogenatmosphere so that they were not exposed to atmospheric air. After that,the operating characteristics of these light-emitting elements weremeasured. The measurement was carried out at room temperature (under anatmosphere in which the temperature was kept at 25° C.).

FIG. 14 shows voltage-luminance characteristics of the light-emittingelements 6 to 9 and the reference light-emitting element 10. In FIG. 14,the horizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 15 shows voltage-currentdensity characteristics of the light-emitting elements 6 to 9 and thereference light-emitting element 10. In FIG. 15, the horizontal axisrepresents voltage (V) and the vertical axis represents current density(mA/cm²). Moreover, Table 4 below shows voltages of the light-emittingelements at a luminance of approximately 1000 cd/m².

TABLE 4 Voltage (V) Light-emitting Element 6 7.6 Light-emitting Element7 7.6 Light-emitting Element 8 7.6 Light-emitting Element 9 7.6Reference Light-emitting Element 10 8.2

FIG. 14 indicates that the light-emitting elements 6 to 9 in each ofwhich the electron-relay layer is provided have a higher luminance thanthe reference light-emitting element 10. In addition, FIG. 15 indicatesthat the light-emitting elements 6 to 9 in each of which theelectron-relay layer is provided have a higher current density than thereference light-emitting element 10.

Thus, it was confirmed that the light-emitting elements 6 to 9 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting elements 6to 9 were light-emitting elements capable of being driven at a lowvoltage.

Example 3

In this example, a light-emitting element that is one embodiment of thepresent invention is described with reference to FIGS. 11A and 11B. Notethat in a light-emitting element and a reference light-emitting elementin this example, for the part that is the same as or similar to thatdescribed in Example 1, the description above is to be referred to.

A method for manufacturing a light-emitting element 11 and a referencelight-emitting element 12 of this example is described below.

First, the light-emitting element 11 is described (see FIG. 11A). Thelight-emitting element 11 of this example was manufactured in a mannersimilar to that of the light-emitting element 1 described in Example 1,except for an electron-transport layer 2103 d of the first EL layer 2103and an electron-injection buffer layer 2104. Therefore, for thestructure and the manufacturing method of the light-emitting element 11,the description in Example 1 is to be referred to, except for theelectron-transport layer 2103 d and the electron-injection buffer layer2104. In the light-emitting element 11 of this example, Alq with athickness of 10 nm and BPhen with a thickness of 20 nm were stacked onthe light-emitting layer 2103 c by evaporation to form anelectron-transport layer 2103 d. Further, lithium oxide (Li₂O) wasevaporated to form the electron-injection buffer layer 2104 to athickness of 0.1 nm on the electron-transport layer 2103 d. Thus, thelight-emitting element 11 of this example was obtained.

Next, the reference light-emitting element 12 is described (see FIG.11B). The reference light-emitting element 12 of this example had thestructure of the light-emitting element 11, from which theelectron-relay layer 2105 was removed. The other layers were formed bymanufacturing methods similar to those of the light-emitting element 11.In the reference light-emitting element 12, after the electron-injectionbuffer layer 2104 was formed, the second charge-generation layer 2106was formed on the electron-injection buffer layer 2104. Thus, thereference light-emitting element 12 of this example was obtained.

Table 5 below shows the element structures of the light-emitting element11 and the reference light-emitting element 12.

TABLE 5 Light-emitting Reference Light- Element 11 emitting Element 122101 ITSO ITSO 110 nm 110 nm 2103 2103a NPB:MoOx NPB:MoOx (=4:1) (=4:1)50 nm 50 nm 2103b NPB NPB 10 nm 10 nm 2103c CzPA:2PCAPA CzPA:2PCAPA(=1:0.05) (=1:0.05) 30 nm 30 nm 2103d Alq Alq 10 nm 1 nm BPhen BPhen 20nm 20 nm 2104 Li₂O Li₂O 0.1 nm 0.1 nm 2105 PTCBI:Li — (=1:0.02) 3 nm2106 NPB:MoOx NPB:MoOx (=4:1) (=4:1) 60 nm 60 nm 2107 2107a NPB NPB 10nm 10 nm 2107b CzPA:2PCAPA CzPA:2PCAPA (=1:0.05) (=1:0.05) 30 nm 30 nm2107c Alq Alq 10 nm 10 nm BPhen Bphen 20 nm 20 nm 2107d LiF LiF 1 nm 1nm 2102 Al Al 200 nm 200 nm

The thus obtained light-emitting element 11 and reference light-emittingelement 12 were sealed in a glove box under a nitrogen atmosphere sothat they were not exposed to atmospheric air. After that, the operatingcharacteristics of these light-emitting elements were measured. Themeasurement was carried out at room temperature (under an atmosphere inwhich the temperature was kept at 25° C.).

FIG. 16 shows voltage-luminance characteristics of the light-emittingelement 11 and the reference light-emitting element 12. In FIG. 16, thehorizontal axis represents applied voltage (V) and the vertical axisrepresents luminance (cd/m²). In addition, FIG. 17 shows voltage-currentdensity characteristics of the light-emitting element 11 and thereference light-emitting element 12. In FIG. 17, the horizontal axisrepresents voltage (V) and the vertical axis represents current density(mA/cm²). Moreover, Table 6 below shows voltages of the light-emittingelements at a luminance of approximately 1000 cd/m².

TABLE 6 Voltage (V) Light-emitting Element 11 7.2 ReferenceLight-emitting Element 12 7.6

FIG. 16 indicates that the light-emitting element 11 in which theelectron-relay layer is provided have a higher luminance than thereference light-emitting element 12. In addition, FIG. 17 indicates thatthe light-emitting element 11 in which the electron-relay layer isprovided have a higher current density than the reference light-emittingelement 12.

Thus, it was confirmed that the light-emitting element 11 of thisexample had characteristics as a light-emitting element and functionedwell. In addition, it was confirmed that the light-emitting element 11was a light-emitting element capable of being driven at a low voltage.

This application is based on Japanese Patent Application serial no.2009-131096 filed with Japan Patent Office on May 29, 2009, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A light-emitting element comprising: ananode; a first light-emitting layer over the anode; a secondlight-emitting layer over the first light-emitting layer; a first layerover the second light-emitting layer, the first layer including a firstdonor material; a second layer over and in direct contact with the firstlayer, the second layer including an electron-transport material; athird layer over and in direct contact with the second layer, the thirdlayer including a hole-transport material and an acceptor material; athird light-emitting layer over the third layer; a fourth light-emittinglayer over the third light-emitting layer; and a cathode over the fourthlight-emitting layer.
 2. The light-emitting element according to claim1, wherein the second layer further includes a second donor material. 3.The light-emitting element according to claim 1, wherein the first donormaterial is selected from the group consisting of an alkali metal, analkaline earth metal, a rare earth metal, an alkali metal compound, analkaline earth metal compound, and a rare earth metal compound.
 4. Thelight-emitting element according to claim 1, wherein the third layer isa layer in which the acceptor material is added so that a weight ratioof the acceptor material to the hole-transport material is greater thanor equal to 0.1:1 and less than or equal to 4.0:1.
 5. The light-emittingelement according to claim 1, wherein the third layer is a stack of alayer including the hole-transport material and a layer including theacceptor material.
 6. The light-emitting element according to claim 1,wherein the acceptor material is an oxide of a metal belonging to Groups4 to 8 of the periodic table.
 7. The light-emitting element according toclaim 1, wherein the acceptor material is a molybdenum oxide.
 8. Thelight-emitting element according to claim 1, wherein luminescent coloremitted from a stacked layer of the first light-emitting layer and thesecond light-emitting layer is different from luminescent color emittedfrom a stacked layer of the third light-emitting layer and the fourthlight-emitting layer.
 9. A light-emitting device manufactured using thelight-emitting element according to claim
 1. 10. An electronic devicecomprising the light-emitting device according to claim
 9. 11. Alighting device comprising the light-emitting device according to claim9.
 12. A light-emitting element comprising: an anode; a firstlight-emitting layer over the anode; a second light-emitting layer overthe first light-emitting layer; a first layer over the secondlight-emitting layer, the first layer including a first donor material;a second layer over the first layer, the second layer including anelectron-transport material; a third layer over the second layer, thethird layer including a hole-transport material and an acceptormaterial; a third light-emitting layer over the third layer; a fourthlight-emitting layer over the third light-emitting layer; and a cathodeover the fourth light-emitting layer, wherein a LUMO level of the secondlayer is between a LUMO level of the second light-emitting layer and anacceptor level of the acceptor material in the third layer.
 13. Thelight-emitting element according to claim 12, wherein the second layerfurther includes a second donor material.
 14. The light-emitting elementaccording to claim 12, wherein the first donor material is selected fromthe group consisting of an alkali metal, an alkaline earth metal, a rareearth metal, an alkali metal compound, an alkaline earth metal compound,and a rare earth metal compound.
 15. The light-emitting elementaccording to claim 12, wherein the third layer is a layer in which theacceptor material is added so that a weight ratio of the acceptormaterial to the hole-transport material is greater than or equal to0.1:1 and less than or equal to 4.0:1.
 16. The light-emitting elementaccording to claim 12, wherein the third layer is a stack of a layerincluding the hole-transport material and a layer including the acceptormaterial.
 17. The light-emitting element according to claim 12, whereinthe acceptor material is an oxide of a metal belonging to Groups 4 to 8of the periodic table.
 18. The light-emitting element according to claim12, wherein the acceptor material is a molybdenum oxide.
 19. Thelight-emitting element according to claim 12, wherein luminescent coloremitted from a stacked layer of the first light-emitting layer and thesecond light-emitting layer is different from luminescent color emittedfrom a stacked layer of the third light-emitting layer and the fourthlight-emitting layer.
 20. A light-emitting device manufactured using thelight-emitting element according to claim
 12. 21. An electronic devicecomprising the light-emitting device according to claim
 20. 22. Alighting device comprising the light-emitting device according to claim20.
 23. The light-emitting element according to claim 12, wherein thesecond layer is over and in contact with the first layer, and whereinthe third layer is over and in contact with the second layer.